Savings and compound interest calculator
See how a lump sum and regular saving grow over time with interest added every month.
- Yearly ISA allowance
- £20,000
- Tax-free interest, basic rate
- £1,000
- Tax-free interest, higher rate
- £500
- £250 a month for 10 years at 4%
- £36,812
- HMRC 2026/27 rates
- Private: nothing leaves your device
- Free, no sign-up
Saving £250 a month at 4%
| After | Paid in | Interest | Total |
|---|---|---|---|
| 5 years | £15,000 | £1,575 | £16,575 |
| 10 years | £30,000 | £6,812 | £36,812 |
| 20 years | £60,000 | £31,694 | £91,694 |
| 30 years | £90,000 | £83,512 | £173,512 |
How £250 a month grows at 4%
Make interest work harder
- Interest in a cash ISA is tax free, up to the £20,000 yearly allowance.
- Outside an ISA, basic rate taxpayers get £1,000 of interest tax free.
- Regular savers often pay more than easy access accounts, on smaller monthly amounts.
The figures assume a fixed rate for the whole period. Real savings rates move with the Bank of England base rate.
Questions people ask
What is compound interest?
Interest paid on your interest. Each month the interest is added to your balance, so the next month's interest is worked out on a slightly bigger amount. Over long periods this makes a big difference.
Do I pay tax on savings interest?
Basic rate taxpayers can earn £1,000 of interest a year tax free under the Personal Savings Allowance, higher rate taxpayers £500. Interest inside a cash ISA is always tax free, up to the £20,000 yearly ISA allowance.
What is AER?
The Annual Equivalent Rate shows what the interest would be over a year if it was compounded, so you can compare accounts that pay monthly with ones that pay yearly.